Systems employing hydraulic drives are utilized for diverse purposes, for example for presses, benders or punch machines. In the context of such applications, on the one hand, exertion of high force at a low speed of the piston (power mode) or of the connected tool (pressing, bending) is required, and on the other hand, a high speed at a low force of the piston (speed mode) or of the connected tool (travel of the tool to/away from the part to be machined) is required. Typically, two separate cylinders are used for this (one driving cylinder for quick movements with low force and one working cylinder for slow movements with high force), each having an actuator, which nowadays is configured as a continuous valve or a variable pump. These actuators require either a high pressure source or an open tank for additional supply of hydraulic fluid for the hydraulic drive. Due to the fixed assignment of one actuator to each driving and working cylinder, the number of required components, the installation effort and the investment costs are enormous. Furthermore, the energy efficiency is insufficient, particularly in partial-load range and when employing continuous valves.
Hydraulic drives with one driving and one working cylinder are known in the art. One such hydraulic drive is, for example, disclosed in JP H06 39285 U and features two mechanically coupled cylinders, two 2/2-way valves and a hydraulic pump, which are all connected with each other within a hydraulic circuit.
EP 2 480 405 B1 discloses a hydraulic drive with one driving cylinder and one working cylinder, with a variable speed pump as actuator in a closed hydraulic circuit, which has a pressure tank connected to it via a valve. The two cylinders are configured as differential cylinders separate from each other. However, a more compact design is desirable. In the arrangement disclosed there, the driving cylinder cannot be utilized as an additional force-exercising component in power mode, so that the force exercised during power mode has to come from the working cylinder alone, which reduces the efficiency of the drive. In speed mode, however, the speed of the tool is exclusively determined by its weight. Thus, in speed mode, no higher speed can be achieved than that which is predetermined by the weight force of the tool. Hence, the variable operation of this hydraulic drive is very limited.
Consequently, it is desirable to provide a hydraulic drive, which requires a minimum number of components, keeps the installation effort low, improves energy efficiency, can be built with a compact design and can be operated with a sufficient degree of variability.